This invention relates to magnetic imaging and more particularly to magnetic imaging with a constant wavelength recording independent of speed variations in a rotational recording member.
In magnetic recording systems conventional to audio and visual recording systems, the magnetic signal is recovered by an electronic "reading" head which reconstructs the desired signal into an appropriate audio or visual electronic signal. These conventional systems depend upon recording the magnetic signals on a magnetizable member in "tracks" which are separated from one another by spacings or "guard bands" of the magnetizable member surface bearing no magnetic signal. These "guard bands" are absolutely necessary so that the "reading" head can accurately reconstruct the desired magnetic signal from the "track" that it is reading without being affected by the fringing magnetic fields from adjacent tracks.
In an imaging system wherein the magnetic latent image is desirably rendered visible with magnetic toner, as opposed to the conventional audio and visual magnetic recording systems, the presence of "guard bands" are highly undesirable because magnetic toner is not attracted thereto. This phenomenon is detrimental to visible imaging schemes relying upon magnetic marking materials to develop a magnetic latent image since it leads to streaks or image deletions, especially in solid areas, upon development of the magnetic latent images.
Amplitude modulated recording conventional to audio magnetic recording and disclosed in U.S. Pat. Nos. 3,013,124; 3,479,036 and 3,275,757 are not desirable for visible development of magnetic latent images. This is so because latent magnetic images for visible development are characterized by very high gamma and "grey" scale is difficult to maintain. Accordingly, an amplitude modulated magnetic image would result in loss of good solid area coverage upon development with a magnetic marking material.
The present invention provides an improved magnetic latent image capable of being rendered visible with magnetic marking material and without image deletion. This obviates the need for guard bands by insuring alignment of pixels in the recording tracks thereby eliminating interference between magnetic fields of adjacent tracks. This is accomplished by mounting an optical encoder on the same shaft upon which is mounted a rotating recording member and generating a signal having a frequency less than the desired recording frequency but proportional to the speed of the rotating recording member. This signal is multiplied in a phase-locked loop circuit to the desired recording frequency and maintained in phase with the lower frequency signal by the phase-locked loop circuit.
Phase-locked loop circuits are known. For example, U.S. Pat. No. 3,634,776 employs an apparatus having a photoconductive cell exposed to a light beam modulated at substantially the same frequency as a reference frequency and receiving electric reference signals from a secondary source, for locking the frequency and phase of an output signal to the reference alternating signal. Therein, the output signal frequency is the same as the reference signal frequency. U.S. Pat. No. 3,600,508 is directed to the reduction of capstan speed in a video tape recorder utilizing the multiplication of control track pulses by a large amount to reduce the degree that they may be out-of-phase with each other. This minimizes the phase difference between control track pulses when capstan speed control is switched from control by control pulses to control by the tachometer.